cpu freeze when changing cell font

cpu freeze when changing cell font

Firefox 1.5.0.4 WinXP SP2
P4 3G HT disabled 1.5G mem

imported an opendocument sheet exported to MSExcel

when changing the font on a few cells at a time, cpu jumps to 100% for a while- 30s to 1m

but tried to change a whole sheet at once and didn't respond for over 30m! ended up killing firefox.

first time using this- is this normal?